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Abstract

Metal-dielectric transitions are important structures that can display a host of optical characteristics including excitation of plasmons. Metal-dielectric discontinuities can furthermore support plasmon excitation without a severe condition on the incident angle of the exciting photons. Using a semi-infinite thin gold film, we study surface plasmon (SP) excitation and the associated electromagnetic near-field distribution by recording the resulting plasmon interference patterns. In particular, we measure interference periods involving SPs at the scanable metal/air interface and the buried metal/glass one. Supported by optical near-field simulations and experiments, we demonstrate that the metal/glass surface plasmon is observable over a wide range of incident angles encompassing values above and below the critical incident angle. As a result, it is shown that scanning near-field microscopy can provide quantitative evaluation of the real part of the buried surface plasmon wavevector.

Figures (4)

Moduli of the complex amplitude of the nth transmitted waves as a function of the normalized wavevectors. Complex amplitudes are obtained by Differential Method for the structure shown in the inset at θ = 40, θSP and 60° (respectively the blue, red and green curve). The different resonant SP modes are marked with the dashed lines. The peak is due to the incident beam and shift within the modification of the incident beam angle. The inset presents the simulation domain and the assumed boundary conditions.

Scheme of the experimental setup. A semi-infinite thin gold film of thickness h≈55 nm is vacuum deposited onto a glass prism. The illumination is carried out at various incident angles θ, by a collimated laser beam preliminary linearly polarized. The optical signal is collected with SNOM in shear force mode via the probe and a photomultiplier tube (PMT). The inset shows the theoretically estimated plasmon resonance angle.

Near-field measurements of a semi-infinite thin gold film. (a) and (b) represent respectively the topographic and the corresponding optical images over a scanned region of 20 µm, including the metal/air edge, for an incident angle of 60 degrees. (c) represents a 20 µm optical image over the gold region for an incident angle of 55 degrees. (d) shows the optical image at the plasmon resonance angle θSP for p-polarization. Under the same conditions, (e) shows the measurement for s-polarization. The profiles are made along the dotted lines represented in the respective images.

Comparison between the calculated (curves) and experimentally measured (symbols) interference periods Λ. Λ1 shows the presence of the metal/air SP and becomes infinite at θSP. Interferences measured associated with Λ2 and Λ3 demonstrate the presence of the SP metal/glass for different angles at the scanable interface. Finally, Λ4 allows a calibration of the experimental setup that confirms the sharpness of our measurements.